The present invention relates to an optical fiber pumped through the cladding and a method of fabricating it.
Optical fibers pumped through the cladding have a particular structure such that they can accept more pump optical power and thereby increase the amplification of the optical signal, which is achieved by absorbing energy.
The amplification energy is supplied by a high-power laser coupled into a cladding around the core of the optical fiber, the luminous energy supplied to the cladding in this way being absorbed by the core as soon as the light rays propagating in the cladding cross the interface between the core and the cladding.
Obviously the efficiency of energy pumping is essentially conditioned by the efficiency of the coupling between the core and the cladding.
One problem that arises with cylindrical core/cladding interfaces is that some light rays conveyed by the cladding follow a helical trajectory around the core without ever impinging on the interface.
The energy conveyed by these rays is therefore injected into the cladding in vain, because it is never used to amplify the signal transported by the core.
To attempt to solve this problem, it has already been proposed, in particular in U.S. Pat. No. 5,966,491, to form longitudinal grooves in the outside surface of the cladding to break up the helical trajectories of the light rays and force them to impinge on the core/cladding interface.
Another solution, proposed in WO 99/30 391, consists in inserting rods into longitudinal passages formed in the cladding to produce radial protuberances at the surface of the cladding and thereby disturb the trajectory of the light rays in the cladding.
The present invention aims to propose an optical fiber with a new structure based on the same principle of disturbing light rays in the cladding.
The present invention provides an optical fiber pumped through the cladding, including:
a core having an optical index n1,
around the core, a first cladding having an index n2 lower than n1, and
around the first cladding, a second cladding having an index n3 lower than n2,
characterized in that the interface between the first cladding and the second cladding has a substantially polygonal cross section.
Obviously, thanks to the polygonal shape of the cross section of the interface, any light ray conveyed by the cladding that might tend to trace out a helix around the core without ever impinging on the core/cladding interface is disturbed by the interface between the first cladding and the second cladding and cannot maintain its helical trajectory.
Consequently, a large number of light rays that would be lost in a circular section structure are recovered by the invention.
In one particular embodiment of the invention, the core consists of doped silica glass, the first cladding consists of undoped or very weakly doped silica glass, and the second cladding consists of silica glass doped negatively, for example with fluorine or boron.
In another embodiment the core consists of strongly doped silica glass, the first cladding consists of silica glass doped positively, for example with germanium, and the second cladding consists of undoped silica glass.
In either of the above two embodiments, the core has the maximum index n1, the first cladding has an index n2 lower than n1, and the second cladding has an index n3 lower than n2.
In one particular embodiment, the fiber includes a low-index polymer coating around its second cladding and the interface between the second cladding and said coating has a substantially polygonal or multilobed cross section.
In this case, a second pumping structure is formed around the first pumping structure consisting of the core and the first cladding.
This second pumping structure includes the first cladding and the second cladding. The interface between the second cladding and the coating has the same function as that previously described for the interface between the first cladding and the second cladding, namely light rays circulating in the second cladding are prevented from tracing out a helix inside the second cladding without ever impinging on the interface between the first cladding and the second cladding, thanks to the substantially polygonal or multilobed shape of the interface between the second cladding and the covering.
Thus it is possible to use power lasers emitting at larger diameters and the energy injected into the first cladding and into the second cladding by the laser is used to amplify the signal conveyed by the core.
The present invention also provides an economic method of fabricating an optical fiber as described above, which has the advantage of not involving costly mechanical machining.
The method is characterized in that it consists in executing the following steps:
placing around a central optical preform including a core having an index n1 surrounded by a first cladding having an index n2 lower than n1, a plurality of rods having an index n3 lower than n2, and
drawing the optical preform and the rods to obtain an optical fiber including a core and two claddings.
In the method according to the invention, the effect of drawing is to compress the rods positioned around the central optical preform to produce the claimed polygonal shape. The polygonal shape results from a balancing of the centripetal forces and plastic deformation of the central optical preform and the rods.
In one particular embodiment, optical preforms with claddings having an index n3 are used as the bars placed around the central optical preform.
An advantage of this embodiment is that it can sometimes be simpler to obtain optical preforms than to obtain homogeneous rods consisting of doped silica glass, for example.
In this case, the core of the preforms used as rods no longer has its usual core function.
In another embodiment of the invention, the central optical preform is one that, if it had been drawn under the usual conditions, would become an optical fiber pumped through the cladding consisting of a core, a first cladding and a second cladding, the whole having a circular section structure, so that a number of light rays circulating helically in the first cladding would not be used.
In this embodiment, the method is characterized in that the central optical preform is, after drawing, an optical fiber pumped through the cladding consisting of a core having an index n1, a first cylindrical cladding of circular section surrounding the core and having an index n2 lower than n1, and a second cylindrical cladding of circular section surrounding the first cladding and having an index n3.
However, thanks to the presence in accordance with the invention of the rods having an index n3 around the central optical preform, drawing said central optical preform and said rods modifies the section of the interface between the first cladding and the second cladding, that section losing its circular symmetry in favor of a polygonal shape because of the balancing of the centripetal forces exerted by the rods.
If the rods are made of the same material as the second cladding of the central optical preform, the second cladding and said tubes fuse and this leads to the formation of a single second cladding.
In one particular embodiment, the central optical preform and the rods having an index n3 are placed in a sleeve within which the atmosphere is controlled for drawing by establishing a vacuum or a partial pressure of neutral gases such as helium or reagents such as C2F6.
The sleeve is advantageously made of silica whose index has been reduced by appropriate doping, for example with fluorine or boron.
This method, well known for drawing optical fibers, is applied without difficulty to the optical fibers according to the present invention.
In another embodiment of the invention, the drawing atmosphere is controlled without recourse to any sleeve. On the other hand, the interstices between the rods having an index n3 are filled and the atmosphere in the volume delimited by the rods is controlled for drawing by establishing a vacuum or a partial pressure of neutral gases such as helium or reagents such as C2F6.
In one particular embodiment, the second cladding is enveloped in a low-index polymer coating.
The presence of the polymer coating produces a third cladding having an index n4 lower than n3 and which channels the light rays in the second cladding to produce a second pumping structure between the third cladding and the second cladding.